Thermoplastic molding compositions comprising polycarbonates and ABS polymers have long been known. DE-A 1 170 141, for example, describes readily processable molding compositions comprising polycarbonates and graft polymers of monomer mixtures of acrylonitrile and an aromatic vinyl hydrocarbon on polybutadiene.
In DE-A 1 810 993, the improved heat resistance of polycarbonate in admixture with ABS graft polymers or copolymers based on α-methylstyrene is emphasised.
The subject-matter of DE-A 22 59 565 and DE-A 23 29 548 is the improved joint line strength of PC/ABS molding compositions, graft polymers of a particular particle size being used in both specifications as a constituent of the ABS component. EP-A 0 704 488 discloses thermoplastic molding compositions having a particle diameter of from 0.20 to 0.35 μm.
DE-A 28 18 679 teaches that PC/ABS mixtures exhibit particularly high low-temperature strength when the ABS polymer contains two graft mixed polymers having different degrees of grafting.
It is also known to filter polymer latices in order to remove impurities or coarse fractions. For example, the filtration of rubber latices is described in Houben Weyl XIV/1, Makromolekulare Stoffe 1, pages 348 to 356 (Georg Thieme Verlag, Stuttgart, 1961) and in DE-A 4 126 483 and U.S. Pat. No. 4,747,959. Connections with the mechanical properties are not known from that prior art.
The object of the present invention is to provide impact-modified compositions having an optimized combination of properties of good surface quality (especially a very low number of faults, so-called “pinholes”), good flow behavior and good stress cracking resistance (ESC behavior). The compositions may also be rendered flame-resistant with flameproofing agents. Impact-modified compositions which have been rendered flame-resistant are suitable especially for thin-walled applications such as notebooks.
It has been found that impact-modified polycarbonate compositions containing a mixture, obtained by co-precipitation, of at least two graft polymers exhibit the desired properties.